Preheat the oven to 250 degrees.
Bring a large pot of water to a boil and cook shells according to directions on box. Drain, rinse with cold water and lay on paper towels to dry.
In a large skillet, cook sausage until no longer pink, Add garlic and chopped spinach and cook 1-2 more minutes until spinach is wilted. Remove from heat.
In a large bowl, add ricotta, egg, oregano, 1/2 cup parmesan and 1 cup of mozzarella. Mix well to combine.
Add sausage-spinach mixture to the cheese mixture and mix well.
In the bottom of a 3-quart casserole dish, add enough sauce to cover the bottom, about 1/2-3/4 cup.
Fill each pasta shell with the spinach mixture and place filled shells in the baking dish in a single file.
Cover shells with the remaining pasta sauce. Sprinkle remaining parmesan and mozzarella evenly over the top.
Cover with foil and bake for 30 minutes. Remove foil and bake another 5-10 minutes until the cheese is melted.